1. Field of the Invention
The present invention relates to an alumina sintered body and a process for producing the same. Said alumina sintered body is suitable for members required to have a corrosion resistance which are mainly in contact with a corrosive solution and gas, or products easily affected by adhesion or adsorption of dust particles of instruments for semiconductor manufacturing, such as vacuum chuck, pincette, and hands used for operations including cleaning, transferring, and surface processing of a silicon wafer; or products required to be pore-free, such as materials for a substrate for a hard disk, a substrate for a magnetic head, and the like, materials for various kinds of industrial mirrors, and the like, and bioceramics materials for artificial tooth, artificial femoral head, and the like.
2. Prior Art
For the constituents in a semiconductor manufacturing process, ceramics materials are generally used for avoiding mixing of metallic elements into a silicon wafer, and contamination. The sintered bodies of alumina and silicon carbide are commonly used as the ceramic materials. The sintered bodies of alumina and zirconia are also used for bioceramics such as artificial tooth, artificial femoral head and artifical nee in view of mixing of metals into a living body, wear resistance, and the like.
The conventional raw material powder such as alumina or silicon carbide in the foregoing applications has a problem that the powder comprises fine particles whose primary particles are 0.5 xcexcm or less and it is non-uniform powder having a spheroidal form or having fractured surfaces. Further, the inside of each particle is also non-homogeneous and have a large number of defects. The powder has a wide particle size distribution and contains a large number of coarse aggregated particles. These cause the formation of pores in a sintering process. As a method for reducing the pores, there have been commonly a method in which several sintering agents are added. However, a sintered body with sufficiently high fired density and less pores cannot be obtained.
The ceramics sintered body product for components of a semiconductor manufacturing system is used by mirror-polishing its portion to be in contact with a silicon wafer. The ceramics sintered body product manufactured from the conventional raw material powder has problems that the wafer is marred by foreign substances such as dust particles adhered to the pore of the sintered body, or the edge of the pore, and the like.
Further, when the silicon wafer or the like supported on the ceramics sintered body having a large number of pores is subjected to processing such as heat treatment or plasma etching, this arises a problem that there occurs falling off of particles, or elution of impurities such as Na from the ceramics in the vicinity of the pores to contaminate the wafer.
The foregoing problems have further become important with the progress of achieving higher density and higher integration of a semiconductor device. Accordingly, a sintered body with high purity and high density, and less pores has been required.
Also, in bioceramics such as artificial tooth, artificial femoral head and artifical nee, the ceramic materials or polymer materials used in a pair therewith are worn out by the edge of the pore in the polished surface. This results in the formation of fine particles to cause pain to a human body. Further, these pores become the starting points of fracture to lower the strength of the sintered body and reliability of the product.
To overcome these problems, there are disclosed plasma-resistant fluorine alumina ceramics and the manufacturing method thereof in Japanese Laid-Open Patent Publication No. 9-2864. In Japanese Laid-Open Patent Publication No. 9-2864, silicon oxide and calcium oxide are added therein in a large amount of 0.3 to 0.7 wt % for reducing the area % of the unsintered particle. Thus, there still has a problem about the corrosion resistance in hot water, an acid solution or an alkaline solution. Further, it is generally known that a sintered body having a small number of pore can be obtained by hot isostatic press. However, there still has a problem industrially that it is required to sinter at high temperatures and high pressures.
The present inventors have found that a high purity alumina sintered body with extremely less pores can be obtained by using the specific alumina powder as a raw material and calcining said alumina in an air atmosphere, and the productivity thereof is high, and have completed the present invention.
That is, the present invention relates to (1) a process for producing a polycrystalline alumina sintered body which comprises the steps of:
subjecting alumina powder to ultrasonic irradiation, mechanical stirring not using a grinding medium, or ultrasonic irradiation and mechanical stirring not using a grinding medium, resulting in slurry dispersed in a solvent;
drying and forming said slurry to produce a molded body; and then sintering said green body in an air atmosphere at a temperature in the range of 1400xc2x0 C. to 1800xc2x0 C.;
wherein said alumina powder having a purity of 99.99 wt % or more comprises a polyhedral particle having substantially no fractured surface, and comprises a alumina particles having polyhedral shape; a D/H ratio of from 0.5 or more to 3.0 or less, wherein D represents a maximum particle diameter parallel to the hexagonal lattice plane of a hexagonal close packed lattice of a alumina, and H represents a maximum particle diameter perpendicular to the hexagonal lattice plane of a hexagonal close packed lattice of a alumina; the number-average particle size of from 0.1 xcexcm or more to 1.0 xcexcM or less; a D90/D10 ratio of 7 or less, wherein D10 and D90 are the particle sizes at 10% cumulation diameter and 90% cumulation diameter, respectively, from the smallest particle side in a cumulative particle size distribution.
Also, the present invention relates to a process described in (1), wherein an alumina powder added a sintering assistant is subjected to ultrasonic irradiation, mechanical stirring not using a grinding medium, or ultrasonic irradiation and mechanical stirring not using a grinding medium, resulting in a slurry dispersed in a solvent.